1. Field of the Invention
The present invention relates to novel refractories with a very high content of alumina.
2. Discussion of Background and Material Information
Refractories containing more than 95% by weight of alumina have been known for many years in the refractory industry, and are employed in large tonnages, for example, in petro-chemical reactors, such as secondary ammonia reactors or reactors for the synthesis of methanol.
Although refractories have high melting points, which are typically above 2,000.degree. C., the maximum temperature of use nevertheless is below 1,500.degree. C. when refractories are employed above this temperature, rapid spalling of the bricks frequently occurs, or the refractories may be crushed under the weight of the pressure exerted by the expansion of masonry at such temperatures. This deficiencies are due to the fact that at temperatures above 1,500.degree. C., the mechanical properties of the refractories deteriorate. For example, the compressive strength falls abruptly from 1,000 bars to values of a few bars, both in an oxidizing atmosphere and in a reducing atmosphere.
In an attempt to overcome these deficiencies, corundum has been bound with mullite binder in the production of refractories. This enables their range of application to be extended to temperatures up to 1,700.degree./1,750.degree. C. in an oxidizing atmosphere.
In a reducing atmosphere, however, the addition of mullite is not possible because silica, when reduced, is volatilized as SiO which renders the corundum bricks spongy and friable. Although carbon-based refractories can then be employed, industrial units which operate above 1,500.degree. C. in a reducing atmosphere are encountered more and more frequently in the evolution of advancing technology, often giving rise to slags as by-products which are rich and readily reducible compounds such as SiO.sub.2, Fe.sub.2 O.sub.3 or TiO.sub.2, so that pure carbon is not particularly suitable as a refractory material. Furthermore, the open porosity of conventional refractories does not drop below 15% in the best of cases, unless a fusible phase is added to them, which has many other disadvantages, the greatest being that it reduces the refractoriness.
Recently, attempts have been made to solve these problems by using graphite, silicon nitride, or sialon as binder. Sialon is a solid solution of alumina in silicon nitride, having the following empirical formula: Si(.sub.6-z) Al.sub.z N.sub.8-x O.sub.z, where z=1 to 4.
Graphite-corundums reach moduli of rupture of 80 to 100 kg/cm.sup.2 at 1,500.degree. C., which is clearly better than those of pure corundum, but is still inadequate for some applications involving mechanical erosion, such as coke beds, ore beds, and the like. Although the porosity of these graphic-corundum refractories can be kept below 15%, typically this involves blocking pores rather than causing a chemical reaction between corundum and graphite. In any any event, graphite-corundum refractories remain highly sensitive to oxidation.
Silicon nitride-corundum or sialon-corundum refractories reach moduli of rupture when hot which are above 200 kg/cm.sup.2 at 1,500.degree. C., but on becoming oxidized they produce silica which readily reacts with iron oxides and other components of slags, forming fusible glasses. Moreover, the nitride or sialon binder tends to decompose thermally above 1,600.degree./1,650.degree. C., which also releases highly reactive silica. As a result, these refractories are rather sensitive to corrosion, to the slags produced in iron steel metallurgy, and to coal ashes above these temperatures. Furthermore, these refractories also cannot be produced economically with a low porosity.
There is, therefore, an unsatisfied need for refractories which are stable at a high temperature in a reducing atmosphere which exhibits high mechanical strength and low porosity, and which do not tend to release silica.